New microbial biofuels could increase efficiency and commercial viability for renewable energy

Although bioethanol and plant oil-derived biodiesel have comprised the first generation of the biofuel industry, their relatively low energy content and incompatibility with existing fuel distribution and storage infrastructure limits their economic use in the future.  However, scientists and engineers are now able to develop more sustainable and economically feasible microbial biofuels through means of metabolic engineering and synthetic biology.  Exploiting the diverse metabolic pathways in organisms such as Saccharomyces cerevisiae and Escherichia coli produces biofuels that have physical properties closely resembling petroleum-derived fuels without requiring additional chemical conversion, which suggests that investigating these new microbial fuels may provide insight into more efficient and commercially viable renewable energy (Rude et al. 2009). Elena Davert 
Rude, A. M., Schirmer, A. New microbial fuels: a biotech perspective. Current Opinion in Microbiology 2009, 12: 274-281.

 All biofuel production involves accessing the energy of the sun stored as chemical energy in the bonds of biologically produced materials through photosynthesis.  Three major pathways to convert renewable resources into energy-rich fuel-like molecules currently exist: 1) direct production by photosynthetic organisms, such as plants or algae; 2) chemical conversion of biomass into fuels; and 3) the fermentative or non-fermentative production by heterotrophic microorganisms such as yeast, fungi, or bacteria. Although the first two options can rely on expensive feedstocks and timely processes, research on key biocatalysts responsible for converting metabolic intermediates into fuel-like molecules hope to increase the economic viability of the third option. 
Microbiologists have started this research process by investigating the metabolic pathways of microorganisms that produce all four types of microbial fuels, which are divided into classes depending on the biological pathway from which they are derived: non-fermentative alcohols, fermentative alcohols, isoprenoid-derived hydrocarbons, and fatty acid-derived hydrocarbons. As is the case with non-microbial biofuel production process, the organic feedstocks involved in microbial fuel production still represent the largest cost component. Because of this, the overall production cost is directly related to the efficiency of the metabolic pathway in converting sugar to fuels. 
In order to effectively examine these efficiencies, the metabolic mass yield, gallon of product per ton of glucose, enthalpy of combustion, and enthalpy of combustion yield was calculated and compared for each microbial fuel pathway and characteristic.  Although ethanol had the highest metabolic mass yield within the microbial gasoline fuels (ethanol, butanol, isobutanol, and 3-methyl-1-butanol), butanol had the highest enthalpy of combustion, making them equally efficient with enthalpy of combustion yields of 97% and 95%.  Although the microbial diesel fuels had slightly lower enthalpies of combustion yields ranging between 75 and 88%, the fatty acid-derived hydrocarbons have the advantage of low solubility in water.  This means that centrifugation can be used to separate these compounds from fermentation broth, as opposed to distillation, which requires much more energy.
Understanding these metabolic pathways will allow for the expansion of the renewable fuel industry as greater knowledge of biocatalysts leads to a greater variety of hydrocarbon product discoveries.  Because the specific metabolic efficiency of any given pathway has a significant impact on the economics of fuel production in a microbial host investing in further research is crucial. Because microbial biofuels are easy to recover and do not require additional chemical conversion, the biofuel production process has the potential to develop into a cost-effective and unsubsidized commercial processes. 

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